This invention relates to a three-dimensional cam, a method for forming such a cam and a grinding apparatus for grinding the cam face of the three-dimensional cam. The cam, method and apparatus are intended particularly for use to operate a cam follower obliquely to the moving direction of a cam face for operating one or more valves associated with a cylinder of an internal combustion engine.
Such a valve operating system is shown in Japanese Published Application JP-A-Hei 12-170881 and corresponding U.S. Pat. No. 6,170,449, entitled xe2x80x9cVALVE OPERATING SYSTEM FOR ENGINExe2x80x9d, issued Jan. 9, 2001 and assigned to the assignee hereof. The arrangement there shown provides an optimum combustion chamber configuration and valve placement where plural valves for a single cylinder can be positioned and operated, each by a single cam and rocker arm follower. However certain improvements are desired in that arrangement and the method and apparatus for forming the operating cam surfaces.
Specifically, the cam face is ground in such a manner that the axis of the grinding wheel is slanted in relation to the axis of the three-dimensional cam. The grinding wheel is moved along the cam profile in the radial direction of the cam while the three-dimensional cam is rotated about its axis at a low speed. The grinding wheel is of a disk-shape with a radius of curvature the same as that of the follower surface. The grinding is performed with the cylindrical outside circumferential surface of the grinding wheel brought into line contact with the cam face in the axial direction over the entire surface of the cam face.
With the resulting three-dimensional cam, a phenomenon might occur that the desired line contact line between the cam surface and the slipper surface is not perfect. Thus a gap is formed after grinding between the cam face and the engaged surface of the follower. This results in a larger contact pressure, which causes abrasion and/or insufficient lubrication. This is more likely if the diameter of the grinding wheel used for grinding is not equal to or smaller than the follower surface.
In addition, since the axis of the grinding wheel is slanted in relation to that of the camshaft during grinding, the length of the camshaft which can be ground on the grinding machine is necessarily limited. This must be done to avoid interference between the camshaft and the grinding machine. Thus only three-dimensional cams for single cylinder engines can be ground on the conventional grinding machine.
It is therefore a principle object of this invention to provide a three-dimensional cam surface that provides sufficient lubrication and avoids high contact pressures as well as a method and apparatus for forming such cam surfaces.
It is another object of this invention to provide a valve operating system utilizing a three-dimensional cam that provides sufficient lubrication and avoids high contact pressures as well as a method and apparatus for forming such a valve operating system.
It is a further object of this invention is to provide a three-dimensional cam grinding machine and method for manufacturing a camshaft for a multi-cylinder engine with a three-dimensional cam.
A first feature of this invention is adapted to be embodied in a three-dimensional cam rotatable about a cam axis for operating a cam follower pivotal about an axis disposed obliquely to the cam axis. The three dimensional cam has an operating face formed with a series of regularly arranged recesses.
A second feature of the invention is adapted to be embodied in an engine valve operating system utilizing such a cam to operate a poppet valve through a respective rocker arm having a follower surface engaged with the cam lobe for pivoting the rocker arm about a pivot axis that is disposed at a skewed angle to the axis of rotation of a camshaft carrying the cam. The rocker arm has an actuating surface engaged with the poppet valve. The poppet valve reciprocates about an axis that is skewed relative to the camshaft rotational axis and which lies in a plane that is generally perpendicular the respective rocker arm pivot axis.
A third feature of the invention is adapted to be embodied in a grinding machine for grinding a three-dimensional slightly concave cam surface. The grinding machine comprises a grinding wheel having a rotational axis and a grinding surface formed by a convex curved surface. The convex curved surface has a radius of curvature smaller than the concave cam surface of the cam. A grinding wheel drive rotates the grinding wheel about its rotational axis and the rotational axis of the grinding wheel is translated based on a target cam profile and a target shape of the cam surface face set for each given rotation angle of the cam. This translation grinds the cam surface along a grinding point that is moved in the rotational and axial directions while maintaining a normal vector of the grinding wheel in coincidence with a normal vector of the desired cam surface.
A fourth feature of the invention is adapted to be embodied in a method of grinding a three-dimensional slightly concave cam surface. This method utilizes a grinding wheel having a rotational axis and a grinding surface formed by a convex curved surface having a radius of curvature smaller than the concave cam surface of the cam. The method comprises the steps of rotating the grinding wheel about its rotational axis and translating the rotational axis of the grinding wheel based on a target cam profile and a target shape of the cam surface face set for each given rotation angle of the cam. This translation grinds the cam surface along a grinding point that is moved in the rotational and axial directions while maintaining a normal vector of the grinding wheel in coincidence with a normal vector of the desired cam surface.